1. Field of the Invention
The invention generally relates to compositions for cleaning contaminant deposits or accumulations from oilfield equipment, and methods of use thereof. For convenience in this description, the contaminant deposits or accumulations are termed "paraffin," paraffinic," and/or "paraffin-like" deposits, which terms are used interchangeably throughout the specification to refer to any material which is insoluble, sparingly soluble, or undispersible in crude oil under conditions of production.
Thus, a given "paraffinic" deposit--in accordance with the terminology herein--may contain high molecular weight aliphatic hydrocarbons (eg., generally having more than about 20 carbon atoms), petroleum resins, asphaltic and asphaltene materials, aromatic hydrocarbons, water, and inorganic matter (such as calcium and magnesium carbonates and sulfates, sand, clay and silicates, rust, iron sulfide, chlorides &c.), and the like. As will be understood by persons ordinarily skilled in the art, the composition of such deposits varies from one crude oil to another, from one field to another and from well to another well in the same field.
2. Prior Art
The accumulation of paraffin on oilfield equipment constitutes an example of precipitation and deposition. Crude oil in the formation is a solution combining the higher molecular weight, paraffinic hydrocarbons with lower molecular weight hydrocarbons which function as solvents. The higher molecular weight, paraffinic solids precipitate whenever the lower molecular weight hydrocarbons transport capacity is reduced. Generally, if the temperature of the crude oil in the formation cools (and/or its pressure decreases), then it can be expected that the higher molecular weight, paraffinic hydrocarbons solids will precipitate and form accumulative deposits.
Paraffin deposition and accumulation is gradual, leading to differential distribution of different chain lengths according to temperature and pressure changes with (i) the fractional amount of the higher-molecular weight paraffin-containing crude oil in the bore, (ii) the pumping thereof, (iii) its transport in conduits, and (iv) its storage in tanks and so on. Consequently, paraffinic deposition and accumulation can and does occur about anywhere in the oil-extraction and -refining process, and plainly is pervasive to oilfield equipment and the associated environment:--it forms and accumulates in such places as on the face of the producing formation, in the casing or the tubing of the well, in oil flowlines and pipelines, in the pump(s), in tank batteries, tank farms and the like.
Paraffin deposits operate to decrease materially the productivity of the wells in which they occur, or decrease materially the capacity of the pipes through which they flow, and so on. Sometimes the deposits accumulate to the point little or no flow can pass through such conduits under normal manner of operation.
The purpose of removing such deposits is obvious. In some areas, wells decline in productivity at a more or less rapid rate because of build-up of such paraffin deposits. In some instances, the decline is sufficient that the wells must be serviced at a frequency ranging from several days to several weeks. In other instances the deposition is slow and hence servicing at longer intervals is sufficient to maintain the well at a satisfactory level of productivity. The same is true of the various conduits through which the oil travels from the well to the refinery:--including through tank batteries, tank farms, pipes and so on. The capacity of a conduit of circular cross-section is reduced greatly by such deposits. The conduit's capacity is frequently reduced to a small fraction of its capacity clean. For example, an operator may find himself burdened with the cost of a 6 or 8 inch (15 or 20 cm) flowline, yet seeing that flowline achieve a capacity equal merely to that of a 3 or 4 inch (8 or 10 cm) Flowline because of paraffinic deposits lining the inside surface of the conduit and constricting the flow area. Ultimately such conduits may be found virtually clogged by such deposits.
Paraffinic deposits likewise occur on formation wells, where this phenomena of inward accretion constricting the flow area is also experienced. If the casing provides nominally an 8 inch (20 cm) diameter flow area when clean, then a 1 inch (2.5 cm) lining of paraffin constricts the effective diameter to 6 inches (15 cm). The effective flow area is reduced by half. If the line pressure were 1,000 psi (70 bar) for line when clean, then the line when fouled and constricted in half will have twice the line pressure, or about 2,000 psi (135 bar). Several aggravating conditions set up when this happens. A local venturi is created, and this promotes erosion of the casing, which sometimes can erode away to allow a leak spot in the casing. Well productivity vis-.alpha.-vis line constriction or leakage may fall substantially to zero, especially in low-pressure fields. As for tank capacities, tank capacities are also reduced by such deposits where a layer may cover the bottom to a depth of several feet.
Various techniques have been employed for the removal of paraffin deposits from oil-producing formations and wells penetrating such formations. These techniques include the use of mechanical and heating devices, explosives, solvents, and so on. Over 50% of the cost of producing and marketing oil from producing wells is related to cleaning procedures to break-up and remove paraffin. The two most common techniques are mechanical scraping and hot-oil flushing. Mechanical scraping, with specially designed instruments, is sometimes also referred to as "knifing." Scraping--where available--is labor-intensive, time-consuming, and generally, expensive. Also, scraping is not available when removal of the paraffin is required from pumps, valves and the like. Hot oil treatments are also expensive because of the considerable cost in the energy-consumption for heating, pumping and the like, and besides, hot oil has a relatively low solvency for paraffin and hence is generally less effective treatment for the dollar. By either foregoing technique/treatment, the removed paraffin usually remains in a solid form or quickly returns to a solid form after treatment.
Chemical methods are effective for dissolving or dispersing paraffin deposits in oil. Normally, paraffin removal is effected through the aid of solvents or surface agents. However it is also known to set up highly-exothermic acid reactions for generating and applying heat to the paraffin solids, but in these cases the paraffin is not truly dissolved and dispersed, and can again quickly return to a solid form after the heat dissipates.
Paraffinic solvents dissolve deposits, but their action is limited, however, to the deposit surface, while surface agents lower surface tension and chemically enwrap small particles, thus changing their adhesion capacity mutually among one another or their adhesion capacity to flowline (eg., pipe) surfaces. Thus broken up, such smaller paraffin particles more easily remain oil-suspended and move freely. The presence of a surface agent lowers the water surface tension breaking the link between paraffin molecules and the flowline (eg., metallic pipeline) wall, inhibiting further paraffin deposition, therefore acting secondarily as a preventative compound or "inhibitor."
Shortcomings associated with known chemical methods include that the known chemical compositions are hazardous materials. They require exceptionally special handling because of toxicity, flammability, or outright explosiveness. Workers are advised to wear gloves, goggles, coveralls, splash aprons and in exceptional instances, respirators or scuba gear to handle the known chemicals, and only within safe proximity of showers and eye rinses and the like. Many popular solvents have such low flashpoints that their handling requires diligence not only in preventing a fire hazard but also in avoiding an explosion. Spills are environmental hazards which require immediate containment and clean-up. Generally, a spill of a gallon or so of the typical known chemical requires excavation and removal of all the soil absorbed with the chemical. Also, the addition of many of the toxic forms of chemicals requires containment and treatment of the removed sludge once it is drawn off the oilfield equipment being treated.
What is needed is a substantially safe product which is substantially non-toxic and non-flammable, and substantially bio-degradable, such that it obviates the special handling attendant to use of the prior art chemicals.